1. Field of the Invention
This invention relates to a bi-phase shift keying signal demodulation circuit for an RDS (Radio Data System) receiver which receives multiplexed data of RDS signals inclusive of digital messages such as information used for selection of broadcasting stations.
2. Description of the Prior Art
As a traffic information system for relieving a traffic snarl, there is known an ARI broadcasting system. In the ARI broadcasting system, a sub-carrier of 57 kHz is constantly multiplexed into an FM radio wave of a broadcasting station so as to identify the station which is broadcasting traffic information. In addition, in that system, a DK signal and a BK signal, obtained by amplitude-modulating the sub-carrier by predetermined frequencies, are multiplexed into the radio wave so as to provide information relating to a beginning or end of the traffic information, or a region in which traffic information is broadcasted.
As a similar system, there is known an RDS system in which digital data identifying broadcasting stations, etc., are multiplexed into an FM radio wave using a sub-carrier of 57 kHz. Data to be multiplexed in the RDS system is configured by a plurality of groups consisting of 104 bits data, and various messages mainly used for the selection of broadcasting station are standardized. The transmission rate of RDS data is 1.1875 kbit/sec, and RDS data is encoded by a differential encoding. Then, a clock signal of 1.1875 kHz is modulated by a bi-phase shift keying using the encoded data. Further, a sub-carrier of 57 kHz is modulated by a carrier suppressed amplitude modulation using modulated data of bi-phase shift keying. Double sideband (DSB) signals of the modulated data are multiplexed to audio data, and the multiplexed data is transmitted. A sub-carrier of RDS data is designed so as to be in phase or in orthogonal phase (shifted by 90 degrees) with the third harmonic of a pilot signal (19 kHz) indicating a stereo broadcasting. When it is required to maintain coexistence of the ARI signal and the RDS modulated signal, the two signals are multiplexed so that the frequencies thereof are equal to each other and the phase relation of them becomes orthogonal with each other (shifted by 90 degrees).
RDS data is obtained by demodulating the RDS modulation signal according to the following manner. Firstly, the RDS modulation signal is extracted from an FM modulation signal using a band pass filter, and a bi-phase shift keying signal is obtained from the extracted signal by a PLL (Phase Locked Loop) synchronization detection. Simultaneously, a synchronization signal of sub-carrier is also generated by the PLL synchronization detection. Then, an RDS clock signal is extracted from the bi-phase shift keying signal and the bi-phase shift keying signal is demodulated using the extracted RDS clock signal. Then, a synchronization detection circuit operates a logical product of the demodulated bi-phase shift keying signal and the synchronizing signal to produce a bi-phase shift keying signal. Then, the RDS data is decoded from the bi-phase shift keying signal by a differential decoding.
In the above described RDS receiver, when the phase relation between the synchronizing signal of the sub-carrier and the sub-carrier component of the bi-phase shift keying modulation signal is stable, the demodulation performance is kept stable. However, in the synchronizing detection, an error component due to jitter is produced when an intensity of the electric field in a received signal is weak. In addition, when an ARI signal is transmitted with an RDS signal, the PLL synchronizing detection circuit locks with the ARI signal whose degree of modulation is higher than that of the RDS signal. In this case, the error component due to jitter increases. As a result, the phase relation between the synchronizing signal and the sub-carrier component of the bi-phase shift keying modulation signal becomes unstable and the RDS data is reproduced erroneously.